Once I sat down to try making my own music toy the first place thing I started trying was using pulsio to play tones as shown in this piezo buzzer learn guide. This is pretty fun, we can edit the frequencies to play different notes, and change sleep times to play faster or slower. The next logical step was to hook up the neotrellis buttons to play some different notes when they are pressed. This was pretty easy due to the neotrellism4 library doing most the heavy lifting and giving us back a nice list of buttons currently pressed.
At this point we've got a super basic little synth with one octave of square wave notes. Pretty darn cool, but is room for improvement. We buzzing a single tone at a time we can't play multiples yet. The volume is pretty loud, up to this point I've been keeping the earbuds out of my ears. We're also only playing out of one of the two audio channels coming out of the 3.5mm audio port on the neotrellis.